Upon arrival tech inspected system and found very low suction pressure and high head pressure in heat mode. Energized reversing valve a few times to switch back and forth cool and heat mode. Originally found a 12 degree temp difference across filter drier. After energizing reversing valve head pressure dropped and suction pressure began to rise. Confirmed no air flow issues, both zones are opening properly, bypass damper is opening as needed when only one zone is running. Filter drier still has a temp difference, suction pressure is still slightly low. Recommended replacing drier and pulling and cleaning outdoor piston while system is open. Customer will hold off on repair as system is still operational as of now. Tech stated issue will more than likely occur again. Sent estimate.